1. Field of the Invention
The present invention relates to an image forming apparatus and an ejection determining method, and more specifically to an image forming apparatus and ejection determining method suitable for determining nozzles with ejection defects in an inkjet head having a plurality of droplet ejection holes (nozzles).
2. Description of the Related Art
An inkjet type image forming apparatus forms images on a recording medium by ejecting ink droplets from nozzles while moving a print head (recording head) in which a plurality of nozzles are arranged and a recording medium relatively with respect to each other. This kind of apparatus may cause the ejection defects such as the obstruction of ink ejection from the nozzles, the defective amount of ink ejected (the dot size deposited on the recording medium), and the defective position to deposit ink, for some reasons according to the increase of the ink viscosity of ink in the head, the infiltration of air bubbles into the ink, and the like.
For this reason, a method in which ejection failures are detected on the basis of the transmissivity of the light by irradiating light, such as laser light, onto droplets ejected from a print head is known in the prior art (referred as to Japanese Patent Application Publication No. 2003-191453).
Japanese Patent Application Publication No. 2003-191453 suggests a method in which the ejection timing of ink in a nozzle group under observation is staggered with respect to other nozzle groups by a time period shorter than the recording ejection cycle according to an inkjet printer comprising a light source and a light receiver for determining ejections.
However, the determination method suggested in Japanese Patent Application Publication No. 2003-191453 is the method to achieve high-speed determination, and the method to control the ejection in units of nozzle groups. Therefore, it is impossible to control the ejection with respect to individual nozzles. Furthermore, in the method disclosed in Japanese Patent Application Publication No. 2003-191453, since the sequence and number of the nozzles to be inspected are limited, it is not possible to carry out inspection in any desired sequence. Even if the number of nozzles to be inspected is limited to a small number, an inspection time equivalent to the time required to inspect all of the nozzles is still needed.
In an embodiment of Japanese Patent Application Publication No. 2003-191453, inspection is carried out in sequence from one end of the nozzle row, and the ejection sequence during determination is predetermined. Therefore, there is little freedom in the ejection sequence and ejection determination cannot be carried out during a printing operation.